1. Field of Invention
The present invention relates to an apparatus and method for making a multi-branching optical coupler.
2. Description of the Related Art
With the rapid progress of optical fiber transmission technology, optical data links, which utilize optical fibers for transmitting data between computers or between a computer and a terminal, have been the subject of much research and development. In configuring such an optical data link, a multi-branching optical coupler, which is capable of mixing light signals from multiple input optical fibers and distributing the mixed light signals evenly and with a low loss to multiple output optical fibers, is an essential device. The method for making such optical star couplers is disclosed in Japanese patent applications, Laid-Open No. 60-24505 and Laid-Open No. 63-205616.
The optical star coupler disclosed in Japanese Patent Application Laid-Open No. 60-24505 is made by inserting an optical fiber bundle into a glass tube, and heating and drawing the glass tube so that the optical fibers are substantially evenly arranged with respect to the center of the coupler. The optical star coupler disclosed in Japanese Patent Application Laid-Open No. 63-205616 is made by coupling multiple 3 dB optical couplers in multi-stages, realizing many branch points.
In the case of the former optical coupler, the optical fibers are arranged symmetrically about the center of the coupler. However, in this case, since no fiber exists at the center, the symmetry with respect to the injection port is poor. As a result, it is difficult to achieve equi-branching characteristics. In the case of the latter optical coupler, unlike a batch fuse type coupler, several couplers have been coupled in multi-stages to form the optical star coupler. As a result, the overall size of the optical star coupler is significantly increased.
Another optical fiber type star coupler disclosed in Japanese Patent Application Laid-Open No. 63-70208 is made by twisting, fusing, and drawing an optical fiber bundle, and covering the optical fiber bundle with a glass tube. However, in this optical fiber type star coupler, the initial arrangement of the fibers is not preserved after the twisting, fusing, and drawing process. As a result, the variance of the intensities of the light output from the fibers is hard to control Therefore, in this case, it is difficult to achieve equi-branching characteristics.
In addition, a multi-branching optical coupler having four branches disclosed in OEC 1994 pp364 is made arranging five fibers on a plane taking symmetry into consideration. In this case, one of these fibers does not contribute to the outputs. This fiber is difficult to produce. Moreover, since the five fibers are arranged horizontally in a row, the width of the coupler is increased. As a result, the size of the coupler case is increased significantly also.
In addition, a method for making a fiber optic coupler and a 1.times.N fiber optic coupler is disclosed in Japanese Patent Application Laid-Open No. H7-140346. According to this method, N fibers are first inserted into a glass tube. The glass tube is then heated and drawn, collapsing the entire glass tube. This method involves a process for putting the fibers into the glass tube, and a process for collapsing the entire glass tube. It is not easy to make these couplers due to these making processes. In addition, it is extremely difficult to collapse the entire glass tube without distorting the fibers. Moreover, the collapsed fibers are deformed, in which case, the light transmission properties of the fibers, for example, the polarized wave dependency, can be adversely affected.